This invention relates to a mechanical press used to convert shells into ends for self-opening cans and the like and, more particularly, to a belt and belt drive for conveying the shells through such a conversion press.
Various beverages, as well as many foods, are packaged in cans formed from aluminum or coated steel. The can body is manufactured by several known methods to include sidewalls, a bottom, a top to which an end is attached after the body is filled. The upper end or top, which may include means by which the can is later opened, is of course manufactured separately. These self-opening ends are made from a shell (the principal component of the end) which is subsequently converted to an end by appropriate scoring and attachment of a tab by known integral riveting techniques.
U.S. Pat. No. Re. 33,061 assigned to the assignee of this application, discloses a typical conversion press for scoring shells and attaching tabs thereto. This press includes a conveyor belt which extends from one side of the press to the other through in-line conversion tooling. Cooperating with the conveyor are upstacker and downstacker mechanisms located on either side of the slide, to supply shells to and remove shells from the conveyor belt. In some applications upstackers may not be used, rather the ends may be discharged from the end of the conveyor belt. The shells are received in circular apertures in the conveyor belt, which is moved stepwise through the press in synchronism with the opening and closing of the tooling.
As shown in FIGS. 3, 6 and 10 of U.S. Pat. No. Re. 33,061 a strip for forming tabs to be attached to the shells is conveyed across the path of the shells in the conveyor belt. The tab strip is conveyed through the press in a generally front to rear direction and tabs are formed in the tab strip as it is conveyed through tab forming stations within the press, while the shells are conveyed simultaneously to successive tooling stations where various forming and scoring operations are performed. The tab strip and shells meet at a tab attachment station where the completed tabs are transferred from the tab strip to the shells to form completed can ends.
In commercial versions of such conversion presses a thin flexible metal conveyor belt, usually made of stainless steel, is often used, and has been provided with a plurality of sprocket holes extending in a longitudinal direction along the conveyor belt. In commercial versions of such presses two or more lanes of shell-receiving apertures or pockets are provided, with the pockets in each lane offset lengthwise of the belt from those pockets in the adjacent lane. This spatial relationship is dictated by the size of the ends and the geometry of the several lanes of tooling in the press, it being understood that the center-to-center distance along each lane between the pockets is the same, and equals the distance the conveyor belt must advance between successive closures of the tooling, to locate the shells concentrically between the successive tooling stations.
A drive drum supporting the conveyor belt at one end thereof has been provided with a plurality of sprocket teeth for engaging in like shaped sprocket holes formed in the belt, thereby to provide positive engagement between the drive drum and the belt for accurately displacing the shells in their intermittent movement through the press.
Conversion presses of this type will have design speeds in the order of 400 to 600 strokes/minute, sometimes even higher. The power for the conveyor drive is usually derived via a power take-off mechanism from the main press drive, wherein one revolution of the main drive is translated into a single stroke of the press tooling. This mechanism is commonly called an "intermitter." To avoid interference between conveyor belt motion and the closing-opening action of the tooling, indexing (incremental advancing) of the conveyor belt is generally confined to about 210 degrees of crankshaft rotation, leaving a dwell of 150 degrees in the conveyor drive, divided around bottom dead center of crank rotation. Thus in a typical beer/beverage conversion press this calls for an indexing motion of the conveyor belt in the order of 3 inches (76.2 mm.), and at a press speed of 600 rev./min. the complete indexing motion must occur in approximately 0.06 seconds. It follows that the forces required to accelerate and decelerate the conveyor system are substantial, and there is considerable stress in the belt.
As a result of these forces being transmitted from the sprocket teeth on the drive drum to the edges of the rectangular sprocket holes in the conveyor belt, stress failures, e.g. cracking occurring at the location of the sprocket holes, have been encountered. When such a failure occurs in a conveyor belt, the press must stop, the belt is cut and removed from the press, then a new belt installed and then welded (in the case of the steel belt) into an endless loop, and the belt drive tightened. The matter of avoiding such stress concentrations by use of a unique pin drive between the drive drum and belt is disclosed in copending U.S. patent application Ser. No. 561,996 filed 26 Jul. 1990.
The metal, usually stainless steel, belts are replaced by cutting completely across the failed belt, removing it from the press, attaching a welding fixture and small arc welder to the press, threading a new belt through the tooling and around the drive and idlers drums, clamping the ends of the new belt in the fixture, then welding the ends of the belt. Suitable mounts are fitted to the press to accommodate such welding fixture, and it in turn provides support for the welder, usually a TIG (tungsten-inert gas) welder of known design. The fixture and welder are readily fitted temporarily to the press and the weld seam completed to provide a the requisite endless belt configuration.
In the process of welding the belts, particularly stainless steel material, it has been found that stress risers are created at the beginning and end of the welded seam, and these lead to stress concentrations in those areas and resultant belt failures. Therefore, it is important to minimize the stress concentrations resulting from such welding process.